The invention relates to a device for demodulating PSK-FM double modulated carrier signals, wherein a carrier wave multiphase-modulated by a main signal is frequency-modulated by a SC (service channel) sub-signal.
In a digital communication link, it is imperative that sub-signals for use in supervising and controlling the link for use in a control line be transmitted through the link separately from the main signals. The transmission systems of the type are generally classified into two categories. In one, sub-signals are transmitted by stuffing pulses into the main signal in a time-division fashion. In the other, a PSK-FM double modulated carrier wave is transmitted in which a carrier wave multiphase-modulated by the main signal is frequency-modulated with a sub-signal. For example, see the publication by Y. Tan et al., "The 8-Level PSK Modem with Cosine Roll-Off Spectrum for Digital Microwave Communications", ICC'76 Conference Record, pp. 29-13 to 29-18.
The first category of the system is adapted for transmitting sub-signals of an improved quality but requires an expensive demodulator for the main signal which must be provided in a repeater station in order to obtain sub-signals when it is otherwise not required that the main signals be demodulated. In the system of the second category in which the modulation system for the main signal differs from that for the sub-signal, a less costly device for demodulating the sub-signals alone may be used in a repeater station. The latter system therefore is advantageous for a long-distance link which requires plural repeater stations. On the transmitting side in the second system, a carrier wave phase-modulated by the main signal and a transmitting local oscillation signal frequency-modulated by the sub-signal are superimposed by an up-frequency converter for transmission. At the receiving side, the main signal is demodulated by a phase-demodulator for the main signal, for example, a phase synchronizing demodulator, and the sub-signal is extracted as an APC (automatic phase control) signal from a carrier wave regenerating PLL (phase-locked loop) circuit included in the phase synchronizing demodulator. This system operates on the principle that the sub-signal is transmitted by FM-modulating the multiphase-modulated carrier wave, that the FM carrier wave is reproduced by a carrier wave regenerating circuit, and that the PLL circuit operates as an FM demodulator. This type of system is described in the publication referenced above and the publication by Y. Tan et al., "2-Ghz Band Digital Radio Equipment Employing 8-Level PSK with Cosine Roll-Off Spectrum Shaping", ICC'78 Conference Record, pp. 33.3.1 to 33.3.5.
Such carrier signal regenerating circuits as used in phase demodulators are largely classified into:
(a) reverse modulation system as shown by U.S. Pat. No. 4,110,706, and W. R. Bennett et al., "Data Transmission", McGraw Hill Book Co., New York, 1965, pp. 253, PA1 (b) a "Costas" system as shown in FIG. 5 of the first-mentioned publication by Y. Tan et al. and U.S. Pat. No. 3,983,499, PA1 (c) multiplying system as shown for example in FIG. 13-1 of the referenced Bennett et al. book.
When employed as demodulators for sub-signals, the systems (a) and (b) cannot reproduce a carrier signal without using a demodulated main signal. Accordingly, these systems disadvantageously require a detection circuit for demodulating a main signal even in the case where demodulation of sub-signals alone is desired as in the case of an IF through repeater station as in the second-mentioned Y. Tan et al. publication. In the system shown in referenced U.S. Pat. No. 4,110,706, a carrier signal component can theoretically be reproduced by 2.sup.N -fold frequency multiplying a 2.sup.N (N=a positive integer)-phase modulated signal. Accordingly, an extra main signal need not be reproduced upon reproduction of the carrier signal unlike in the systems (a) and (b). In the multiphase modulation, particularly for N.gtoreq.3 in which a transmitted signal is subjected to strict bandwidth limitations as in the roll-off shaping, the proportion of correct phase information in the time slot of a carrier signal drastically decreases due to intersymbol interference casued by the bandwidth limitation. This results in an appreciable lowering of the ratio of desired carrier signal components after multiplication due to undesirable clutter components.
Increases in phase states result in increases in the number of multiplications required, as well as requiring a multiplier having a wider bandwidth, as the bit rate of the main signal increases. Under such conditions, difficulties are encountered, because of multiplier limitations, in employing reproduced carrier signals as carrier signals for demodulating the main signal as well as the sub-signal unless a number of additional circuits are provided, as disclosed in U.S. Pat. No. 3,835,404. However, as the circuit disclosed in this patent includes a sampling circuit and a limiter in the stages after the multiplier, the bandwidth problems of the multiplier are not ameliorated. None of the portions, in which the ratio of desired signals to undesirable signals is lowered because of the multiplier limitations, can be removed by sampling. For example, the portions of time containing cross-talk in which undesired clutter components arise during portions containing no phase information, as in the vicinity of a phase-change point or in the vicinity of a data-change point, are multiplied and the components thus multiplied exert an influence upon the portions containing phase information, namely the phase-arranged portions. A carrier signal reproducing circuit may be provided for reproducing a carrier signal of an improved quality for demodulation of the main signal, as referenced in U.S. Pat. No. 3,835,404. However, no description is given as to reproduction of sub-signals from a PSK-FM double-modulated carrier signal.